The present invention is generally directed to band-gap reference circuits, and more specifically, to a low power, low noise, fast startup, 1-volt operation band-gap reference circuit using second order curvature correction.
Band-gap circuits are well known devices that are used to provide a reference voltage that is relatively constant across a wide temperature range. Exemplary band-gap circuits are disclosed in U.S. Pat. No. 3,887,863 and U.S. Pat. No. 6,278,320. The disclosures of U.S. Pat. Nos. 3,887,863 and 6,278,320 are hereby incorporated by reference into the present disclosure as if fully set forth herein.
The theory of operation of band-gap reference circuits is well known in the art. Two different sized base-emitter diodes are biased with the same current level. Since the diodes are the same size, the diodes operate in different current density. The differences in current density are used to generate a proportional-to-absolute-temperature (PTAT) current. The PTAT current develops a voltage across a resistor, thereby creating a PTAT voltage. The PTAT voltage is proportional to absolute temperature and has a positive temperature coefficient. This voltage is then summed to a base-emitter junction voltage of a diode that has a negative temperature coefficient. The negative temperature coefficient and the positive temperature coefficient cancel each other out, so that the combined voltage across the resistor and the base-emitter junction is constant over temperature.
FIG. 1 illustrates conventional band-gap reference circuit 100 according to an exemplary embodiment of the prior art. Band-gap reference circuit 100 comprises capacitor 195, current sources 110 and 115, amplifiers 120 and 125, N-channel transistors 131-133, resistors 140 and 145, PNP bipolar junction transistors 151-153, amplifier 160, P-channel transistor 165, and resistor 170. PNP bipolar junction transistors 151-153 are connected as diodes and are referred to hereafter as PNP diodes 151-153. According to an exemplary embodiment, PNP diode 151 has an area that is eight times larger than the area of PNP diode 152 (i.e., 8:1 ratio).
Current sources 110 and 115 are current mirrors that generate identical currents I1 and I2, respectively. Amplifier 120 samples the voltage on the drain of N-channel transistor 131, a high impedance node. Amplifier 125 converts the output of amplifier 120 to a control voltage that is applied to the gates of N-channel transistors 131-133. The control voltage forces transistors 131 and 132,to deliver equal currents I1 and I2 to PNP diodes 151 and 152, respectively. Capacitor 105 sets the dominant pole of the feedback loop formed by amplifiers 120 and 125 and N-channel transistor 131.
A temperature independent band-gap reference voltage, V(bg), is established by summing the voltage across a resistor (having a positive temperature coefficient) and the base-emitter voltage, V(be), of a pn junction of a pnp diode having negative temperature coefficient. Typically, the sizes of the pnp diodes are chosen with an 8:1 area ratios (the result of using common centroid matching geometry throughout the industry), as in the case of PNP diodes 151 and 152, so that the PNP diodes operate at unequal current densities.
Let:
1) PNP diode 151 be denoted as D1;
2) PNP diode 152 be denoted as D2; and
3) PNP diode 153 be denoted as D3.
From FIG. 1 it can be seen that:
V(be)D2=V(be)D1+I1(Ri),xe2x80x83xe2x80x83[Eqn. 1]
where Ri is the resistance value of resistor 140.
The current, i, in a PNP diode is given by the equation:
xe2x80x83i=IS(eV(be)/V1),xe2x80x83xe2x80x83[Eqn. 2]
where i is proportional to area. Rearranging terms in Equation 2 gives:
V(be)=VT[ln(i/IS)].xe2x80x83xe2x80x83[Eqn. 3]
Substituting V(be) in Equation 3 into Equation 1 gives the expression:
V(be)D2xe2x88x92V(be)D1=I1(Ri)=VT[ln(8iD1/iD1],xe2x80x83xe2x80x83[Eqn. 4]
where iD1 is the current in D1 (i.e., PNP diode 151) and iD2 is the current in D2 (i.e., PNP diode 152). Since iD1 and iD2 are equal, Equation 4 reduces to:
I1(Ri)=VT(ln 8)xe2x80x83xe2x80x83[Eqn. 5]
Thus, the current I1 in PNP diode 151 is:
I1=VT(ln 8)/Ri.xe2x80x83xe2x80x83[Eqn. 6]
It is noted that VT, the thermal voltage has a positive temperature coefficient, VT=+26 mV, at room temperature. Thus, the current I1 is proportional to absolute temperature (PTAT).
The current I1 is mirrored by the current I3 in N-channel transistor 133. The current I3 may be used to establish a band-gap reference voltage, V(bg) for use in biasing, where:
V(bg)=I3(k*Rr)+V(be)D3.xe2x80x83xe2x80x83[Eqn. 7]
By selecting a suitable multiplier, k, such that dV(bg)/dT=0, V(bg) becomes independent of temperature.
Furthermore, it is possible to generate a reference current, I4, that is proportional to V(bg). This is achieved by the feedback loop formed by amplifier 160, P-channel transistor 165 and resistor 170, which generate I4=V(bg)/Ro, where Ro is the resistance value of resistor 170.
As FIG. 1 shows, the band-gap circuit provides a temperature compensated reference voltage output for use by other circuits in a system. A temperature insensitive, high-tolerance band-gap reference circuit is an indispensable building block in modern chip level integrated circuits (ICs). Band-gap reference circuits are used for biasing analog circuits, as a reference level for data converters, to set trip points for comparators and sensors, and the like.
Some applications, such as data converters and low drop-out (LDO) voltage regulators, require low-noise characteristics and a high PSRR (power supply rejection ratio). Prior art devices may employ large value filter capacitor to improve noise and PSRR performance. However, this impacts system cost and board size and, worst of all, slows down turn-on time (i.e., the time it takes for the band-gap reference circuit to stabilize the output voltage after being turned on). For example, many cellular telephones conserve battery power by periodically turning off various circuit-blocks. If the turn-on time is too long, it is not practical to shut off these circuits. This wastes power and impacts system performance. Since band-gap reference circuits are relatively slow to startup, it is necessary that a faster startup technique be incorporated to meet the current needs of cellular telephone and other similar power critical applications.
As mentioned, conventional band-gap reference circuit 100 consumes a relatively large amount of current ( greater than 100 microamperes) and is slow to start up ( greater than 100 microseconds). Additionally, many modern portable applications, such as cellular telephones and pagers, operate from a +1.2 power supply rail. The V(be) base-emitter voltage drops in band-gap reference circuit 100 leave very little voltage margin with which to operate.
Furthermore, the current (i) in a PNP diode, as defined in Equation 2, exhibits non-linear behavior at high temperature. This is a key element that leads to large variation of band-gap voltage over temperature. Reducing such a variation often requires the introduction of a suitable correction current. Prior art current correction devices require elaborate circuitry and trimming techniques to generate an appropriate non-linear correction current that mitigates the nonlinear behavior of the PNP diode current at high temperature. The result is a flatter band-gap voltage profile over temperature.
Therefore, there is a need in the art for an improved band-gap reference circuit that is capable of operating from a low voltage (e.g., +1.2 volts) power supply rail. More particularly, there is a band-gap reference circuit that uses a simple circuit to generate an appropriate non-linear correction current to correct the nonlinear behavior of the PNP diode current at high temperature.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide an improved band-gap reference circuit. According to an advantageous embodiment of the present invention, the band-gap reference circuit comprises: 1) a first current source for generating a first reference current; 2) a first circuit branch for receiving a portion of the first reference current, the first circuit branch comprising a first resistor having a positive temperature coefficient connected in series with a base-emitter junction of a first PNP diode having a negative temperature coefficient, wherein an emitter current of the first PNP diode develops a first combined voltage across the series connection of the first resistor and the base-emitter junction of the first PNP diode; 3) a comparison circuit for comparing the first combined voltage to a base-emitter voltage of a second PNP diode and, in response to the comparison, adjusting a band-gap reference voltage; and 4) a correction current generating circuit capable of injecting a correction current into an emitter of the second PNP diode, wherein the injected correction current at least partially offsets a non-linear drop-off in the band-gap reference voltage caused by the second PNP diode as temperature increases.
According to one embodiment of the present invention, the band-gap reference circuit further comprises a second current source for generating a second reference current equal to the first reference current, wherein the emitter of the second PNP diode receives at least a portion of the second reference current.
According to another embodiment of the present invention, the correction current generating circuit comprises a first biased-off P-channel transistor, wherein a first leakage current of the first biased-off P-channel transistor comprises at least a portion of the correction current.
According to still another embodiment of the present invention, the first leakage current increases non-linearly as temperature increases.
According to yet another embodiment of the present invention, the correction current generating circuit comprises a second biased-off P-channel transistor, wherein a second leakage current of the second biased-off P-channel transistor comprises at least a portion of the correction current.
According to a further embodiment of the present invention, the second leakage current increases non-linearly as temperature increases.
According to a still further embodiment of the present invention, the band-gap reference circuit further comprises a correction current control circuit for combining the first and second leakage currents to form the correction current.
According to a yet further embodiment of the present invention, the correction current control circuit combines the first and second leakage currents according to a process corner of the band-gap reference circuit.
Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9cor,xe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases.